Known in the present state of the art are the L-threonine producing strains of microorganisms of a variety of species (e.g., Brevibacterium flavum, Serratia mercescens, Escherichia coli, and others). It is the mutating strains of E. coli whose cells contain hybrid plasmids carrying the genes of the threonine operon (U.S. Pat. Nos. 4,278,785; 4,321,325) that prove to be the most efficacious L-threonine producers, of which the most productive is Escherichia coli strain VNIIgenetika M-1 (U.S. Pat. No. 4,321,325), which contains multicopy plasmid pYN7 obtained on the base of vector pBR322 and incorporating the threonine operon of E. coli strain K12 resistant to alpha-amino-beta-hydroxyvaleric acid, an analogue of threonine. The genes of the threonine operon of plasmid pYN7 code a bifunctional enzyme, viz., aspartate-kinase-homoserinedehydrogenase, which is insensitive to inhibition with L-threonine. Said strain M-1 is capable of accumulating L-threonine till a concentration of 30 g/l for a 40-hour fermentation period in laboratory fermenters when cultivated under conditions of feeding a sugar-ammonia additive to the nutrient medium in response to a signal sent by the pH sensor.
The aforesaid strain is featured by low productivity and inadequate stability of the plasmid, which compels one to make use of antibiotics to retain the plasmid in course of fermentation.